Plasmonic colours on bulk metals: laser colouring of large areas exhibiting high topography

The use of metal nanostructures to produce colour has recently attracted a great deal of interest. This interest is motivated by colours that can last a long time and that can be rendered down to the diffraction limit, and by processes that avoid the use of inks, paints or pigments for environmental, health or other reasons. The central idea consists of forming metal nanostructures which exhibit plasmon resonances in the visible such that the spectrum of reflected light renders a desired colour. We describe a single-step laser-writing process that produces a full palette of colours on bulk metal objects. The colours are rendered through spectral subtraction of incident white light. Surface plasmons on networks of metal nanoparticles created by laser ablation play a central role in the colour rendition. The plasmonic nature of the colours are studied via large-scale finite-difference time-domain simulations based on the statistical analysis of the nanoparticle distribution. The process is demonstrated on Ag, Au, Cu and Al surfaces, and on minted Ag coins targeting the collectibles market. We also discuss the use of these coloured surfaces in plasmonic assisted photochemistry and their passivation for day-to-day use. Reactions on silver that are normally driven by UV light exposure are demonstrated to occur in the visible spectrum.